1. Field of the Invention
The present invention relates to an arc tube which for ms a main portion of a discharge bulb that is used for a light source of a vehicle headlamp or the like. More particularly, the present invention relates to a mercury-free arc tube for a discharge bulb in which mercury is not contained in a closed glass sphere that serves as a discharge light-emitting portion of the arc tube.
2. Related Art
Generally, a discharge bulb has been used for a light source of a vehicle headlamp or the like. For example, as disclosed in JP-A-06-020645, this kind of discharge bulb has a structure in which it includes an arc tube main body having a closed glass sphere that serves as a discharge light-emitting portion in which electrodes are oppositely provided, and a cylindrical shroud glass tube that is airtightly integrated with the arc tube main body so as to surround the closed glass sphere. In the cylindrical shroud glass tube that surrounds the closed glass sphere, air (or nitrogen) is sealed (filled).
In addition, in the discharge bulb, mercury, together with an inert gas and metal halides, is generally sealed in the closed glass sphere of the arc tube main body so as to increase a light-emitting efficiency, as is also described in JP-A-06-020645. In recent years, however, there has been a heightened social need for reducing the use of mercury, which is an environmentally harmful substance. As a result, a so-called mercury-free arc tube has been developed in which mercury is not sealed in a closed glass sphere.
If mercury is sealed in the closed grass sphere, it is possible to obtain high vapor pressure even at a low temperature in comparison with other metals, ant the mercury acts as a thermal buffer with respect to a tube wall of the arc tube in the surroundings of the arc formed between the electrodes. However, in the mercury-free arc tube, since mercury does not exist (that is, a thermal buffering function of mercury does not exist), the temperature of the tube wall of the arc tube undesirably becomes high. Then, in the mercury-free arc tube, the heat of the closed glass sphere serving as the discharge light-emitting portion is transmitted to the shroud glass tube through the air surrounding the closed glass sphere (or nitrogen), so that the heat loss becomes large correspondingly. Thus, there is a problem in that a light-emitting efficiency of the arc tube becomes lowered.
In addition, since the surface temperature of the shroud glass tube rises due to the heat transfer from the closed glass sphere serving as the discharge light-emitting portion, there is another problem in that silicon gas or the like in a lighting device becomes attached to the surface of the shroud glass tube, and thus the shroud glass becomes whitened.
Accordingly, as described in JP-A-2004-063158, in order to resolve the above-mentioned problems (in that a light-emitting efficiency of the arc light source becomes lowered, and the shroud glass tube becomes whitened), sealed is gas that contains any one of Ar, Kr, and Xe, each having relatively lower thermal conductivity than air, by at least 50% in the shroud glass tube that surrounds the closed glass sphere, such that the thermal conductivity is remarkably lowered in a heat insulating space around the closed glass sphere formed by the shroud glass tube.
In the mercury-free arc tube of JP-A-2004-063158, the above-mentioned problems, in that a light-emitting efficiency of the arc tube becomes lowered and the shroud glass tube becomes whitened, are resolved, because the thermal conductivity is remarkably lowered in a heat insulating space around the closed glass sphere formed by the shroud glass tube surrounding the closed glass sphere. However, since the thermal conductivity is markedly lowered in the heat insulating space around the closed glass sphere formed by the shroud glass tube surrounding the closed glass sphere, the temperature in the closed glass sphere excessively rises, and flickering (flickering of arc) occurs with devitrification of an inner wall. As a result, a life span of the arc tube becomes shortened, which results in lowering performance.
Further, a maximum value of an outer diameter the shroud glass tube is regulated in a standard (ECER99). So, an inner diameter of the shroud grass cannot be large, since some inner thickness of the glass of the shroud grass is necessary in order to ensure the strength. In addition, since the closed glass sphere becomes a high temperature, it is necessary to increase an outer diameter of the closed glass sphere so as to ensure durability of the closed glass sphere. Therefore, the gap between the closed glass sphere and the shroud glass tube is set to 1 mm or less in a conventional discharge bulb.
The mercury-free arc tube is manufactured in a state in which a center axis of the shroud glass tube aligns with a discharge axis between electrodes (hereinafter, referred to as discharge axis). However, in the process of manufacturing the arc tube, the center axis and the discharge axis of the shroud glass tube may not accurately align with each other, and thus may deviate from each other (the gap around the closed glass sphere is not uniform in a circumferential direction). In addition, the arc tube, in which the center axis and the discharge axis of the shroud glass tube deviate from each other, is installed in the insulating plug unit so as to form the discharge bulb. When the discharge axis deviates from the center axis of the shroud glass tube in a downward direction of the center axis (when the center axis of the closed glass sphere deviates downward from the center axis of the shroud glass tube, and thus the minute gap δ1 between the shroud glass tube and a lower portion of the closed glass sphere is smaller than the minute gap δ2 between the shroud glass tube and an upper portion of the closed glass sphere), there is a problem in that the light flux of the arc tube becomes lowered. The reason why the light flux of the arc tube becomes lowered is considered as follows. That is, in the large heat insulating space above the closed glass sphere (minute gap δ2), the heat propagation from the closed glass sphere to the shroud glass tube is suppressed. In contrast, in the small heat insulating space below the closed glass sphere (minute gap δ1), the heat propagation from the closed glass sphere to the shroud glass tube is accelerated. As a result, since radiation of the heat is accelerated from the lower region of the shroud glass tube, the temperature of the cold point in the closed glass sphere rises and the vapor pressure rises, which results in increasing a light flux.